The research will employ a variety of psychophysical procedures in which the independent variable consists of some controllable aspect of the acoustic (and/or visual) field and the dependent variable consists of one or more measurable aspects of the rat's overt startle pattern. The research will encompass four areas of investigation: (1) an analysis of stimulus change as a sufficient condition to modify startle (2) an examination of cross modality effects in startle (3) an investigation of the role of attention-like processes in the modification of startle (4) an analysis of the anatomical aspects of the startle reflex. The work is designed to identify the variables that influence startle so as to further clarify the role of the reaction in the behavioral repertoire. At present, the accumulated evidence points to the conclusion that the startle reaction and the orienting reflex are intimately associated. The research proposed here will help to specify that relationship.